This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-8737 filed on Jan. 18, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a detector for detecting a concentration of pollutant gas such as NOx using an oxide-semiconductor.
2. Description of Related Art
A gas concentration detector of this kind is disclosed in JP-A-11-240323. In this detector, a detected concentration level is compared with a calculated comparison level, and it is determined that a gas concentration is high when the ratio of the detected level to the comparison level is higher than a predetermined ratio. The comparison level is calculated to follows the detected level with a certain delay time when the detected concentration level increases at a rate exceeding a predetermined rate, so that the ratio of the detected level to the comparison level becomes sufficiently high. When it is determined that the gas concentration is high, operation of an automobile air conditioner is switched from an outside air mode to an inside air mode.
It is known that there are two regions in detecting the gas concentration, one is a stable region where the detected concentration level changes according to an actual concentration and the other is an unstable region where the detected level abruptly changes independently from the actual concentration. In the aforementioned detector, however, the gas concentration is detected without differentiating two regions. Accordingly, the detector cannot accurately detect the gas concentration in the, unstable region for the reason described below.
The detector includes a gas sensor composed of an oxide-semiconductor as a gas-sensitive element and a heater for heating the oxide-semiconductor. When the oxide-semiconductor is not heated, moisture and oxygen are chemically absorbed on its surface. When it is heated, moisture and oxygen are removed from the surface, thereby bringing the surface to a state where excessive electrons present on the surface. Oxygen adjacent to the surface is attracted by the electrons and absorbed to the surface (referred to as negative charge absorption), making a potential barrier between semiconductor molecules high and thereby increasing an electrical resistance of the oxide-semiconductor. Since the gas concentration is detected based on the resistance, the detected level increases. This creates the unstable region where the detected level rapidly increases independently from an actual concentration level. After the detected level reaches a certain level, the stable region appears, where the detected level changes according to the actual concentration level.
If the comparison level is set based on the operation in the stable region, commonly to the stable region and the unstable region, as is in the aforementioned conventional detector, the actual concentration level cannot be detected by comparing the detected level with the comparison level. That is, the ratio of the detected level to the comparison level exceeds a predetermined ratio, even when the actual concentration level is not high.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved gas concentration detector which is able to detect the actual gas concentration even in the unstable region.
The gas concentration detector of the present invention detects a concentration of pollutant gas such as NOx or SOx contained in ambient air. The detector is composed of an oxide-semiconductor gas sensor heated by an electric heater and a controller for determining the gas concentration based on a detected concentration level and a calculated comparison level. The controller includes a microcomputer, a timer and other components. The oxide-semiconductor gas sensor absorbs moisture when it is not heated. Until the gas sensor is sufficiently heated to remove the absorbed moisture and the oxygen attraction by negative charges is saturated, there is an unstable region where the gas concentration is not correctly detected. After that, the gas sensor enters a stable region where the gas concentration is correctly detected.
The controller calculates a comparison level with which the detected level is compared to determine the gas concentration. The comparison level is calculated so that it follows the detected level with a certain delay. When a ratio of the detected level to the comparison level becomes higher than a certain value, or when a difference between the two levels exceeds a predetermined value, the detector determines that the gas concentration is high. Though the gas concentration is correctly determined in this manner in the stable region, it cannot be determined in the unstable region. To make it possible to determine the gas concentration in the unstable region, the delay of the comparison level to the detected level is made smaller than the delay in the stable region, thereby making the comparison level quickly follows the detected level.
The gas sensor periodically outputs the detected level with a certain interval, and the controller calculates the comparison level with the same interval. A present comparison level Bgs(n) is calculated according to the following formula: Bgs(n)=Bgs(nxe2x88x921)+[Vgs(n)xe2x88x92Bgs(nxe2x88x921)]/m, where Bgs(nxe2x88x921) is a previous comparison level, Vgs(n) is a present detected level and m is a time constant larger than one. Accordingly, the delay in the unstable region can be made smaller by making time constant m smaller compared with that in the stable region.
The gas concentration detector may be used in an automobile air conditioner operated under an inside air mode or an outside air mode. When the detector determines that the pollutant gas concentration in the outside air is high, the air conditioner operation is switched from the outside air mode to the inside air mode.
To determine the duration of the unstable region, a heater-operated time for supplying heat to the oxide-semiconductor gas sensor is measured by the timer in the controller. Further, to estimate an amount of moisture absorbed to the oxide-semiconductor gas sensor while the sensor is not energized, the non-energized time may be measured by the timer. The time constant m is set according to the non-energized time, so that the comparison level follows the detected level more quickly when a higher amount of moisture is estimated to be absorbed.
According to the present invention, the gas concentration in ambient air is effectively determined even in the unstable region of the oxide-semiconductor gas sensor.